(NaturalNews) Women with high blood levels of the naturally occurring plant chemicals known as carotenoids have significantly lower risk of breast cancer, according to a study conducted by researchers from Brigham & Women's Hospital and Harvard Medical School and published in the Journal of the National Cancer Institute.

It has been well-established that a diet high in fruits and vegetables significantly lowers the risk of a wide variety of cancers, as well as providing a number of other health benefits. Often, the most colorful vegetables and fruits are among the healthiest, due to the pigments that they contain.

The carotenoids are pigments that give plans deep yellow, orange or red colors. They include alpha- and beta-carotene, lycopene, lutein and zeaxanthin. Foods high in carotenoids (particularly beta-carotene, alpha-carotene, and beta-cryptoxanthin) include carrots, sweet potatoes, tomatoes, winter squash, apricots, mangoes and papayas. Despite their color, green leafy vegetables are also high in carotenoids, particularly beta-carotene and lutein - the chlorophyll in their leaves merely masks the orange color beneath. Tomatoes, guava and pink grapefruit are also high in lycopene.

The researchers conducted a meta-analysis of the data from 8 separate studies on a total of 7,000 women - consisting of 80 percent of all published data on the link between carotenoids and breast cancer. In addition, the researchers re-analyzed all the original blood samples in order to standardize measurements of carotenoid levels.

They found that women whose blood was in the top 20 percent in terms of carotenoid levels were 15 to 20 percent less likely to develop breast cancer than women whose blood was in the bottom 20 percent.

"It looks like it is a linear relationship," lead author A. Heather Eliassen said. "The higher you go, the [lower] your risk is. There is some benefit at a moderate level of carotenoids and there is even more benefit at a higher level."

The link between higher blood levels and lower cancer risk was highest for cancers classified as estrogen-receptor-negative, which are among the most aggressive, lethal and hard-to-treat breast cancers.

Turn to natural sources, not supplements

The reason for the connection was unclear, but the researchers believe that the body may metabolize carotenoids into retinol, which plays an important role in gene expression and cell growth, and may also hamper tumor growth. Carotenoids may also help cells communicate with each other, and may boost the immune system's ability to fight tumors on its own.

Eliassen warned that the best way to increase carotenoid levels in the blood is through food, not supplements.

"We are not at a point to recommend supplements," she said. "We know from other studies that certain supplements can increase the risk of lung cancer among smokers. We are not going to go in that direction clearly, but increasing fruit and vegetable intake clearly can provide lots of health benefits and may also reduce the risk of breast cancer."

Because carotenoids are fat-soluble, they should ideally be consumed in meals with at least a small amount of fat, such as an olive-oil-based salad dressing.